The present invention relates to an apparatus for the digital measurement of an angle, the legs of which are materialized or defined by two mechanical parts, hereafter called stator and rotor respectively, which are rotably mounted with respect to each other about a common axis or shaft. A disk rotating about the same shaft with constant angular velocity supports a circular graduation made up of equidistant radial lines and gaps. Lines and gaps for example, may be distinguished by any mechanical, optical, electrical or magnetical property and can be sensed by appropriate sensors or pick-up elements. At least one stator-fixed pick-up and at least one rotor fixed pick-up, provide one or more pairs of ac-signals, the phase difference of which varies by 2.pi., while the rotor revolves by one graduation interval with respect to the stator.
The measurement of this phase difference gives an interpolation of the graduation interval (fine measurement), whilst the number of entire intervals contained in the angle to be measured is still unknown.
Constructions are known in which, apart from the circular graduation the rotating disk contains an individual reference mark, which is scanned by two additional pick-up elements, one of which being stator-fixed and the other rotor-fixed. The still unknown number of entire graduation intervals (course measurement) is obtained by counting the cycles of one graduation signal between successive passages of the zero mark on the stator-fixed and rotor-fixed pick-up elements.
In addition, solutions are known in which the zero mark is integrated into the graduation and is realized by omitting one graduation mark, obviating the additional pick-up elements. However, additional expense is incurred to reconstruct the missing signal cycle for the fine measurement.
For the purpose of the course measurement, the reference mark must appear once on the stator-fixed pick-up element and once on the rotor-fixed pick-up. This involves an observation time which, in the least favorable case, involves a complete rotation of the disk circle, which is prejudicial in certain cases. In the case of a theodolite-like instrument, the same evaluation electronics must alternately measure two angles about different axes (azimuth and elevation). This must take place in a rapid sequence when surveying (approximately 2 to 4 complete measurements per second). This leads to the wish of obtaining the coarse information from a fraction of a complete disk rotation.